AimlessWanderer
Remember to forget me!
I know how to make shims of course but that's not the same as knowing how to make really good shims.
Put a blade in, do one pass, but don't rinse the razor. Leave the lather on to dry, and finish your shave with an alternate razor and blade. The next day, there'll be a clear line between the soap scum on the part of the blade that overhung the support, and the clean area that was sat flush on the rails
That dividing line is where to cut, and you'll have a shim more or less the perfect width. Once you have one the right width, trimming others is easier, and you won't need to do the part shave thing again.